Stainless steel is steel alloyed with chromium, nickel and other elements. Materials referred to as stainless steel generally contain a minimum of 12% chromium. Its outer surface exposed to air forms a protective oxide coating. However, contrary to popular belief, stainless steel will corrode. It needs oxygen to maintain its protective capability and if there is no oxygen, stainless steel will corrode.
If stainless steel is to be coated with a black coating for non-reflective purposes, the stainless steel must be "activated" before the black coating will form on it. This requires removal of the protective oxide coating. This may be done by dipping it into an acid bath, usually sulfuric or hydrocloric acid. The activated stainless steel is then coated by emmersion in a blackening bath.
Cast stainless steel also requires corrosion-proofing. This steel is heat treated to get a desired high strength material. It corrodes readily due to the formation of an iron rich as-cast surface that readily corrodes when exposed to air.